Complex cleaning system for heat exchanger

ABSTRACT

The purpose of the present disclosure is to solve the problems of a difficult operation or occurrence of corrosion damage to equipment caused by the attachment of ammonium sulfate salt, which is generated when unreacted ammonia (NH 3  slip) and sulfur trioxide (SO 3 ) in exhaust gas are bonded with each other when a selective catalytic reduction (SCR) is used to eliminate nitrogen oxides that are contained in the exhaust gas generated during the combustion of a boiler or the like, to a heat exchanger of an air preheater (APH) or the like installed at the rear of an SCR device and blocks a passage of the exhaust gas such that the pressure inside the boiler is increased. To this end, a dry ice cleaning device is installed at the front of an exhaust gas inlet in an air preheater such that the blocking of a heat exchanger caused by ammonium sulfate salt or the like is removed by spraying dry ice pellets, and at the same time, high-temperature steam spraying device is installed at a cold end of the air preheater so as to spray the steam in the same direction as that of air supplied to a boiler, thereby removing contaminants more effectively. Furthermore, in order to effectively prevent blocking which occurs in a cold end of the air preheater, dry ice is sprayed to the front of an exhaust gas inlet of the air preheater and also, to the front of a supply air inlet of the air preheater which is an opposite direction thereof, such that a cleaning effect is increased.

TECHNICAL FIELD

The present disclosure relates to a method of cleaning a heat exchanger, and more particularly, to a complex cleaning system for the heat exchanger, which is to solve the problems of a difficult operation or shutdown caused by the attachment of ammonium bisulfate, which is generated when unreacted ammonia and sulfur trioxide in exhaust gas are bonded with each other when a selective catalytic reduction (SCR) method is used to eliminate nitrogen oxide that are contained in the exhaust gas generated during combustion of a boiler or the like, to a heat exchanger of an air preheater or the like installed at the rear of the SCR device and blocks a passage of the exhaust gas.

BACKGROUND ART

Nitrogen oxide, as a pollutant causing environmental harm, which is contained in the exhaust gas generated when coal, oil, gas, or combustible materials, etc. burn in a combustion engine, should be eliminated before being exhausted in air.

To eliminate nitrogen oxide contained in the exhaust gas, a selective non-catalytic reduction (SNCR) method directly spraying a reducer such as ammonia into a boiler, or a selective catalytic reduction (SCR) method spraying a reducer at the rear of the boiler, has been used. The SCR method is mainly used because the SNCR method has low denitrification efficiency.

The SCR method is a method to convert nitride oxide (NO_(x)) contained in the exhaust gas into nitrogen and water by passing through catalyst after blending nitride oxide with the reducer such as ammonia.

FIG. 1 illustrates a general layout of an antipollution control plant used in a power plant or a combustion engine such as an industrial boiler. According to FIG. 1, exhaust gas discharged from the boiler passes through a coal economizer, a catalytic layer of a selective non-catalytic reduction (SCR) device, an air preheater, an electrostatic precipitator, and a flue-gas desulfurizer, sequentially, to be exhausted via a chimney.

Generally, the exhaust gas generated by combustion of coal or heavy oil in the boiler contains sulfur dioxide (SO₂) and sulfur trioxide (SO₃). Sulfur dioxide is partially oxidized to sulfur trioxide during passing through denitrification catalyst, as the following chemical equation 1, and as such, a concentration of sulfur trioxide in the exhaust gas is increased after passing through the SCR device.

2SO₂+O₂→2SO₃  [Chemical Equation 1]

Meanwhile, water exists in the exhaust gas, while ammonia supplied to the SCR device is partially reacted with sulfur trioxide and water to form ammonium bisulfate, as the following chemical equation 2.

NH₃+SO₃+H₂O→NH₄HSO₄  [Chemical Equation 2]

Ammonium bisulfate decreases activity of catalyst, corrodes the rear facilities of the SCR device, and increases loss of pressure in the boiler due to blocking a hole for catalyst and a passage for the exhaust gas in the heat exchanger. For this reason, during operation of the SCR device, a discharge concentration of unreacted ammonia is normally limited to be equal to or smaller than 2 to 3 ppm. However, in spite of this limitation, during operation of the SCR device, a blocking problem in heat exchangers of various equipments occurs frequently.

To this end, in some power plants, a real-time high-pressure water cleaning device is further provided at the heat exchanger, particularly the air preheater, which includes a soot blower as shown in FIG. 2, to eliminate the blockage periodically. Spraying the high-pressure water, however, damages a coating layer of the thermal element, and as such, the blocking problem in the air preheater becomes severe. This may not be an effective cleaning method. Generally, although the high-pressure water cleaning device has very high operation pressure of 150 kg/cm² g to 200 kg/cm² g, in a case of the severe blocking problem in the air preheater, even the pressure of 300 kg/cm² g may be not enough to solve the blocking problem.

Furthermore, as shown in FIG. 2, when cleaning by using the high-pressure water, plenty of water may flow in following devices such as the electrostatic precipitator to damage them. To prevent damage of the following elements, as shown FIG. 3, a drainage system may be further provided. In this case, however, the damage by water may be not prevented fundamentally.

To this end, the same inventors as this disclosure had disclosed a dry ice cleaning device installed at the front of an exhaust gas inlet of an air preheater to eliminate pollutants by spraying dry ice pellets. (KR Patent Application Publication No. 10-2011-0096603) FIG. 4 is a block diagram illustrating a system for eliminating ammonium bisulfate in the air preheater using the dry ice cleaning device disclosed in KR Patent Application Publication No 10-2011-0096603.

FIG. 5 shows a process for eliminating ammonium bisulfate attached on the heat element using the dry ice pellets in the dry ice cleaning device of FIG. 4. The dry ice pellets are sprayed at a high speed with air of a high-pressure (or a low-pressure) from the dry ice cleaning device to hit the surface of the thermal element of the air preheater. The dry ice pellets rapidly freezes ammonium bisulfate attached on the thermal element at ultralow temperature (for example, −78° C.), and then ammonium bisulfate contracts due to a temperature difference between ammonium bisulfate and ambient air, thereby forming numbers of cracks thereof. The dry ice pellets permeate into the cracks of ammonium bisulfate while sublimating to expand equal to or greater than 800 times in its volume, thereby lifting to separate only ammonium bisulfate from the surface of the thermal element. As described above, contaminants frozen cryogenically are easily separated from the surface by a wind-pressure of the dry ice cleaning device to be discharged through the rear of the air preheater.

When only ammonium bisulfate exists as a contaminant, the contaminant is attached at a middle part of the air preheater, thereby being easily removed only using the dry ice pellets. However, when the other contaminants occur due to a decline of fuel quality, it may be hard to remove these contaminants. In particular, when the ambient temperature extremely drops due to severe cold in winter season, as shown in FIG. 6, the contaminants, in which ammonium bisulfate bonded to dust is frozen with water, block an exhaust gas path at the cold end of the air preheater. Accordingly, it is ineffective to remove the contaminants by only using the cleaning technique with the dry ice pellets.

REFERENCES OF THE RELATED ART Patent Document 1

Korea Patent Laid-open Publication No. 10-2011-0096603

Technical Problem

As described above, the present disclosure provides a complex cleaning method to effectively remove contaminants from a thermal element, while solving the problems of a damage of the thermal element by spraying high-pressure water and decline of performance and durability of an electric precipitator connected to the rear of an air preheater, due to high content of water contained in an exhaust gas.

Technical Solution

The present disclosure may be applied to various industrial plants and combustion engines using SCR, more particular power plants or industrial boilers, but is not limited thereto. Hereafter, an air preheater as a heat exchanger will be described as an example, but the present disclosure is not intended to be limited thereto.

When a thermal element of an air preheater is contaminated by only ammonium bisulfate, a layer of ammonium bisulfate is mainly attached at a middle of the air preheater, thereby easily removing the layer of ammonium bisulfate only using dry ice pellets. When various contaminants, however, are accumulated naturally or due to a decline of fuel quality, it is hard to eliminate the contaminants. In particular, when an ambient temperature extremely drops due to severe cold in winter season, it often happens that ammonium bisulfate bonded to dust is frozen with water at the cold end of the air preheater, and as such, it is necessary to complement the cleaning by the sole dry ice.

To this end, the illustrated embodiment of the present disclosure provides a method of effectively eliminating the contaminants attached on the thermal element of the heat exchanger, by alternately or simultaneously operating a high-temperature steam spraying device and a dry ice spraying device, which are installed.

Furthermore, according to the illustrated embodiment of the present disclosure, dry ice and high-temperature steam are sprayed to not only an exhaust gas inlet, which is an original spraying position but also a supply air inlet, namely a cold end, in an opposite spraying direction with respect to an original spraying direction, thereby improving cleaning effect.

In an aspect of the present disclosure, the present invention provides the complex cleaning method for the air preheater to effectively remove the contaminants, for example, ammonium bisulfate and dust, formed on a surface of the air preheater by alternately or simultaneously spraying the high-temperature steam and spraying the dry ice to the air preheater. In more detail, the complex cleaning method according to the illustrated embodiment of the present disclosure includes spraying the high-temperature steam to the air preheater at a temperature of 90° C. to 500° C., preferably, 90° C. to 400° C. and a pressure of 10 kg/cm² g to 30 kg/cm² g, preferably, 20 kg/cm² g; spraying the dry ice pellets to the inlet of the air preheater in parallel with a surface of the thermal element at a pressure of 0.5 kg/cm² g to 20 kg/cm² g and a speed of 200 m/sec and 400 m/sec, while each dry ice pellet having a diameter equal to or smaller than 3 mm, preferably, between 0.1 mm and 3 mm; and removing the contaminants formed on the thermal element.

Preferably, when the temperature is equal to or smaller than 400° C. and the pressure is equal to or smaller than 20 kg/cm² g in the described method, time-worn of the air preheater may be minimized and the contaminants may be effectively eliminated.

Preferably, when cleaning by the steam, the high-temperature steam spraying device is installed at the cold end of the air preheater so as to spray the steam to the air preheater in the same direction 9 as a direction of air supplied to the boiler, or in a direction opposite to the direction of air supplied to the boiler toward the exhaust gas inlet of the air preheater. Meanwhile, when cleaning by the dry ice pellets, the dry ice spraying device is installed at the front of the exhaust gas inlet of the air preheater so as to spray the dry ice to the air preheater in the same direction 8 as a flow direction of the exhaust gas, or in a direction opposite to the flow direction of the exhaust gas toward a supply air inlet.

More preferably, the steam is sprayed to the cold end of the air preheater in the same direction 9 as the direction of the supply air and the dry ice pellets are sprayed to the air preheater in a direction of the exhaust gas supplied to the air preheater, namely, the direction 8 of the exhaust gas.

Cleaning using the steam and cleaning using the dry ice pellets may be simultaneously performed. Alternatively, after cleaning using the steam, cleaning using the dry ice pellets may be performed, whereas after cleaning using the dry ice pellets, cleaning using the steam may be sequentially performed.

According to the present disclosure, when the dry ice pellets are sprayed to the air preheater, the dry ice pellets hit the thermal element of the air preheater to be smashed, and then ammonium bisulfate covered on the surface of the thermal element is rapidly frozen by particles of smashed dry ice pellets at a temperature range of 0° C. to −78.5° C., and as such, cracks occurs in the layer of ammonium bisulfate. In this case, the smashed dry ice pellet particles penetrate into the cracks in the layer of ammonium bisulfate while the dry ice particles sublimate, thereby ammonium bisulfate is separated from the thermal element. The other contaminants such as dust accumulated on the surface of the thermal element of the air preheater are separated by the same process described above, thereby cleaning of the thermal element is performed.

Advantageous Effects

The complex cleaning method according to the present disclosure may be conveniently performed during operation mode, namely, without suspension of plant operation which was a prior problem, and do not damage an air preheater or an electrostatic precipitator. Furthermore, spraying high-temperature steam has a lower pressure than that of spraying high-pressure water, and water amount for spraying the high-temperature steam is very small compared with spraying the high-pressure water, and as such, water content in an exhaust gas is not increased. Accordingly, additional waste water disposal facilities are not required because contaminants are not discharged additionally.

Meanwhile, CO₂ collected from the exhaust gas of the power plant may be recycled to generate dry ice, thereby being advantageous in terms of utilization of CO₂ and cost reduction.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a layout of a general power plant or boiler installed conventional antipollution control facilities wherein a coal economizer provided at the rear of the boiler, a selective catalytic reduction (SCR) provided at the rear of the coal economizer, and an air preheater provided at the rear of the SCR.

FIG. 2 illustrates a conventional system for eliminating ammonium bisulfate in an air preheater using a soot blower and a water washing system.

FIG. 3 illustrates a drain system for eliminating water during water washing.

FIG. 4 illustrates a conventional system for eliminating ammonium bisulfate in an air preheater including a dry ice cleaning device instead of the water washing system shown in FIG. 2.

FIG. 5 illustrates a process for eliminating ammonium bisulfate using dry ice pellets.

FIG. 6 illustrates blocking an exhaust gas path at a cold end of the air preheater, with ammonium bisulfate coupled to dust frozen with water when ambient temperature rapidly drops in winter season.

FIG. 7 illustrates a system operating a dry ice cleaning device and a high-temperature steam spraying system which are installed together.

DETAILED DESCRIPTION OF MAIN ELEMENTS

1: rotation direction of air preheater, 2: air preheater, 3: dry ice pellet spraying nozzle, 4: dry ice pellet spraying device, 5: dry ice pellet sizer, 6: high-temperature steam spraying device, 7: supply air outlet, 8: exhaust gas inlet, 9: supply air inlet, 10: exhaust gas outlet

BEST MODE

Hereinafter, the embodiments of the present disclosure will be described in detail with reference to accompanying drawings.

As shown in FIG. 5, dry ice pellets are sprayed from a cleaning device by air having a high-pressure or low-pressure to hit a surface of a thermal element of an air preheater. The dry ice pellets causes ammonium bisulfate to be rapidly frozen at ultralow temperature (−78° C.), such that ammonium bisulfate contracts due to a temperature difference with ambient temperature, thereby a number of cracks are occurred. The dry ice pellets penetrate into ammonium bisulfate through the cracks while sublimating to expand equal to or greater than 800 times in its volume, thereby lifting to remove only ammonium bisulfate. According to the same process, dust or the other contaminants as well as ammonium bisulfate are separated to be eliminated and materials frozen at ultralow-temperature are easily separated to be discharged to the rear of the air preheater.

FIG. 7 shows a system improving the conventional dry ice cleaning device of FIG. 4, as adding a high-temperature steam spraying device 6 having two spraying positions, an exhaust gas inlet, namely the original spraying position, and a supply air inlet, namely a cold end of the air preheater. In the high-temperature steam spraying device 6, as a steam temperature and pressure are increased, cleaning efficiency is excellent, whereas it is possible to damage the thermal element, such that the steam is sprayed at a temperature equal to or smaller than 400° C. and a pressure equal to or smaller than 20 kg/cm² g. The steam has a pressure at about a tenth part of an operation pressure of high-pressure water, for example 150 kg/cm² g to 200 kg/cm² g, in the conventional cleaning using the high-pressure water, and as such the thermal element is not damaged compared with the conventional cleaning.

Although method of cleaning the air preheater has been disclosed according to the embodiments of the present disclosure in detail with reference to the accompanying drawings, it will be understood that these embodiments are illustrative and that the scope of the inventive subject matter is not limited to them.

MODE FOR DISCLOSURE

Various embodiments have been described in the best mode for carrying out the disclosure. 

1. A method of complex cleaning for a heat exchanger comprising: spraying high-temperature steam to the heat exchanger at a temperature of 90° C. to 500° C. and a pressure of 10 kg/cm² g to 30 kg/cm² g; spraying dry ice pellets to an inlet of the heat exchanger in parallel with a surface of a thermal element at a pressure of 0.5 kg/cm² g to 20 kg/cm² g and a speed of 200 m/sec to 400 m/sec, each dry ice pellet having a diameter of 0.1 mm to 3 mm; and eliminating contaminants formed on the surface of the thermal element, wherein, in spraying the high-temperature steam, the high-temperature steam is sprayed to an exhaust gas inlet of the heat exchanger or to a supply air inlet of the heat exchanger opposite to the exhaust gas inlet; and in spraying the dry ice pellets, the dry ice pellets are sprayed to the exhaust gas inlet of the heat exchanger or to the supply air inlet of the heat exchanger opposite to the exhaust gas inlet.
 2. The method according to claim 1, wherein spraying the high-temperature steam and spraying the dry ice pellets are performed simultaneously or sequentially.
 3. The method according to claim 1, wherein spraying the dry ice pellets comprises: rapidly freezing a layer of ammonium bisulfate covered on the surface of the thermal element of the heat exchanger at 0° C. to −78.5° C. using dry ice pellet particles shattered by hitting the thermal element to have cracks in the layer of ammonium bisulfate; infiltrating the shattered dry ice pellet particles into the cracks of the layer of ammonium bisulfate; and separating and removing ammonium bisulfate from the surface of the thermal element by sublimation of the dry ice particles.
 4. The method according to claim 1, wherein the heat exchanger is an air preheater. 